The broad objectives of this program are to explore the molecular and physiological functions of ion transport in microorganisms. There is now strong evidence that in bacteria the linkage between transport systems and cellular metabolism is usually indirect, effected by the circulation of protons across the cytoplasmic membrane (chemiosmotic theory). More recently it has become clear that bacteria also possess an array of transport systems that depend on the cells' ATP supply. Many ion transport systems, including those for K+ and (in some organisms) Na+ and Pi, are ATPlinked. The nature and mechanism of these systems are the primary focus of this research. Research will be conducted along two lines. The major line concerns ion transport in the Gram-positive bacterium Streptococcus faecalis, which has both technical and conceptual advantages. We will pursue our discovery that sodium is expelled by an ATPdriven electroneutral pump (probably by exchange for H+), by means of concurrent experiments with intact cells and everted membrane vesicles. Subsequently, emphasis will shift to potassium transport which is known to require the collaboration of ATP and the protonmotive force but whose mechanism remains elusive. The second line of research is concerned with ion transport in the eukaryotic water mold Blastocladiella emersonii. Electrophysiological studies presently underway indicate that in this organism the membrane potential arises by K+ diffusion, and point to a novel pattern of chemiosmotic ion currents. I hope to explore both the genesis of the ion circulation and its possible role in the transport of nutrients through the rhizoid filaments to the thallus.